The present invention relates to a method for the preparation of angular or polyhedral particles of a rare earth ammonium double oxalate as a precursor of a rare earth oxide used in a wide field of applications as a base material of various kinds of phosphors, sintering aid of ceramic materials and so on.
It is a well established process for the preparation of a rare earth ammonium double oxalate that an aqueous solution of a water-soluble salt of a rare earth element is admixed with an ammonium compound and oxalic acid or ammonium oxalate so as to precipitate the double oxalate represented by the general formula NH.sub.4 R(C.sub.2 O.sub.4).sub.2 .cndot.nH.sub.2 O, in which R is an atom of the rare earth element and n is a positive number. The precipitated particles of the double oxalate, which are separated from the aqueous medium, washed with water, dried and calcined to give a rare earth oxide, may have different particle configuration or morphology depending on the conditions of the precipitation reaction while it is known that the double oxalate as a precursor of a rare earth oxide should desirably have an angular or polyhedral particle configuration.
While rare earth oxides are widely used in various applications, it is desirable that the rare earth oxide particles have a particle configuration and particle size as uniform as possible in order for the rare earth oxide powder to be imparted with greatly enhanced utilizability. In this regard, the method of calcination of a rare earth ammonium double oxalate to give an oxide is a desirable process but this method is not always quite satisfactory with poor reproducibility in respect of the uniformity of the particle configuration and particle size depending on the conditions of the precipitation reaction.